Conditions of dementia such as AD are frequently characterised by a progressive accumulation of intracellular and/or extracellular deposits of proteinaceous structures such as β-amyloid plaques and neurofibrillary tangles (NFTs) in the brains of affected patients. The appearance of these lesions largely correlates with pathological neurofibrillary degeneration and brain atrophy, as well as with cognitive impairment (Mukaetova-Ladinska, E. B. et al. (2000) Am. J. Pathol. Vol. 157, No. 2, 623-636).
In AD, both neuritic plaques and NFTs contain paired helical filaments (PHFs), of which a major constituent is the microtubule-associated protein tau (Wischik et al. (1988a) PNAS USA 85, 4506-4510). Plaques also contain extracellular β-amyloid fibrils derived from the abnormal processing of amyloid precursor protein (APP; Kang et al. (1987) Nature 325, 733). An article by Wischik et al. (in ‘Neurobiology of Alzheimer's Disease’, 2nd Edition (2000) Eds. Dawbarn, D. and Allen, S. J., The Molecular and Cellular Neurobiology Series, Bios Scientific Publishers, Oxford) discusses in detail the putative role of tau protein in the pathogenesis of neurodegenerative dementias. Loss of the normal form of tau, accumulation of pathological PHFs and loss of synapses in the mid-frontal cortex all correlate with associated cognitive impairment. Furthermore, loss of synapses and loss of pyramidal cells both correlate with morphometric measures of tau-reactive neurofibrillary pathology, which parallels, at a molecular level, an almost total redistribution of the tau protein pool from a soluble to a polymerised form (i.e. PHFs) in Alzheimer's disease.
Tau exists in alternatively-spliced isoforms, which contain three or four copies of a repeat sequence corresponding to the microtubule-binding domain (Goedert, M., et al. (1989) EMBO J. 8, 393-399; Goedert, M., et al. (1989) Neuron 3, 519-526). Tau in PHFs is proteolytically processed to a core domain (Wischik, C. M., et al. (1988b) PNAS. USA 85, 4884-4888; Wischik et al. (1988a) Loc cit.); Novak, M., et al. (1993) EMBO J. 12, 365-370) which is composed of a phase-shifted version of the repeat domain; only three repeats are involved in the stable tau-tau interaction (Jakes, R., et al. (1991) EMBO J. 10, 2725-2729). Once formed, PHF-like tau aggregates act as seeds for the further capture and provide a template for proteolytic processing of full-length tau protein (Wischik et al. 1996 Proc Natl Acad Sci USA 93, 11213-11218).
The phase shift which is observed in the repeat domain of tau incorporated into PHFs suggests that the repeat domain undergoes an induced conformational change during incorporation into the filament. During the onset of AD, it is envisaged that this conformational change could be initiated by the binding of tau to a pathological substrate, such as damaged or mutated membrane proteins (see Wischik, C. M., et al. (1997) in “Microtubule-associated proteins: modifications in disease”, eds. Avila, J., Brandt, R. and Kosik, K. S. (Harwood Academic Publishers, Amsterdam) pp. 185-241).
In the course of their formation and accumulation, PHFs first assemble to form amorphous aggregates within the cytoplasm, probably from early tau oligomers which become truncated prior to, or in the course of, PHF assembly (Mena, R., et al. (1995) Acta Neuropathol. 89, 50-56; Mena, R., et al. (1996) Acta Neuropathol. 91, 633-641). These filaments then go on to form classical intracellular NFTs. In this state, the PHFs consist of a core of truncated tau and a fuzzy outer coat containing full-length tau (Wischik., C. M., et al, (1996) loc. cit.). The assembly process is exponential, consuming the cellular pool of normal functional tau and inducing new tau synthesis to make up the deficit (Lai, R. Y. K., et al., (1995), Neurobiology of Ageing, Vol. 16, No. 3, 433-445). Eventually, functional impairment of the neurone progresses to the point of cell death, leaving behind an extracellular NFT. Cell death is highly correlated with the number of extracellular NFTs (Wischik et al. 2000, loc.cit). As tangles are extruded into the extracellular space, there is progressive loss of the fuzzy outer coat of the neurone with corresponding loss of N-terminal tau immunoreactivity, but preservation of tau immunoreactivity associated with the PHF core (Bondareff, W. et al., (1994) J. Neuropath. Exper. Neurol., Vol. 53, No. 2, 158-164).
Clearly the identification of compounds that could modulate the aggregation of disease-associated proteins such as tau is of great interest.
WO 96/30766 (F Hoffman-La Roche) discloses assays for the inhibition of tau-tau association, and certain inhibitors identified using the assays. FIGS. 23 and 24 therein rank certain compounds according to their inhibitory properties. Vitamin K (=K2) has a value of 0.674 and menadione (also known as Vitamin K3) is denoted as having a value of 1.042. In the ranking a value of 1 represents binding equivalent to that observed in the absence of compound.
Of course, vitamin K is well known, per se, as a therapeutic. A brief overview of Vitamin K is given in “Goodman and Gilman's The Pharmacological Basis of Therapeutics”, 9th edition, pp 1582-1585, 1998. More comprehensive reviews are provided in William Friedrich, “Vitamins”, pp 285-338, 1988; Thorp et al (1995), Drugs 49, 376-387; Vermeer and Schurgers (2000), Blood Stasis and Thrombosis, 14, 339-353. The reduced form of vitamin K acts as a cofactor for the enzyme gamma-glutamyl carboxylase. This enzyme is responsible for the conversion of glutamic acid residues to gamma-carboxyglutamate on the vitamin K—dependent clotting factors (factors II, VII, IX, X and the anticoagulation proteins, protein C and protein S). Other gamma-carboxyglutamic acid containing proteins (so called Gla-proteins) have been found in plasma (protein Z), bone (osteocalcin), kidney, lung and testicular tissue. The functions of non-haematological Gla-proteins are outlined in Vermeer and Schurgers (2000, loc cit.). However such proteins are not found in the brain (Vermeer (1990), Biochem J, 266, 625-636).
Traditional therapeutic uses of Vitamin K analogues include hypoprothrominaemia in adults and the newborn, inadequate absorption of lipid-soluble substances, and intestinal malabsorbtion syndromes such as cystic fibrosis, sprue, Crohn's disease and enterocolitis.
In addition to the therapeutic uses described above, vitamin K3 is also known to have anti-tumour activity in vitro against a broad range of rodent and human tumour cell lines (Hu et al., 1996). The mechanism of this activity is not known. It has been shown that vitamin K3 has complex effects on several second messenger kinase cascades (Markovits et al., 1998; Wu and Sun 1999), and it has been proposed specifically that vitamin K3 forms a covalent bond with kinases/phsophatases containing the peptide sequence (I/V)HCXXXXXR(S/T)G inducing cell-cycle arrest and cell death by inhibitng Cdc25 phosphatase. However the consensus sequence [HCXXXXXR(S/T)G] is not found in the repeat domain of tau.
One study (Nakajima et al., 1993) examined the effects of vitamin K derivatives on cultured CNS neurones and found that vitamins K1 and K2 had prominent survival promoting effects in the range 10 nM—1 μM. By contrast, vitamin K3 (menadione) had only ˜10% of this survival promoting activity, and this only at 1 μM. Whatever the mechanism of this effect, it was not dependent on the vitamin K cycle, since coumarin anticoagulant which interferes with epoxide reductase step had no effect on the survival promotion assay. Using cultured human neuroblastoma cells, Ko et al. (1997) showed that menadione at high doses (200 μM) caused both prominent dephosphorylation of tau protein, and oxidation of a broad range of proteins. Interestingly, for the reasons discussed in detail in Wischik et al. (2000), tau protein dephosphorylation might be expected to enhance tau protein aggregation.
More recently, Ko et al. (2000) discusses the role of pathogenic mutations in alpha-synuclein in sensitising neuronal cells to oxidative stress induced by high dose menadione. In this paper, the authors argue that thiol-depletion induced by compounds which generate oxidative-stress is a general mechanism responsible for toxicity of mutant alpha-synuclein in hereditary Parkinson's disease, with the implication that rational approaches to therapy would be based on counteracting the oxidant damage produced by substances such as menadione.
However, apart from the isolated data given in WO 96/30766 (F Hoffman-La Roche), no investigation has been carried out to demonstrate and optimise a role for napthoquinone-type compounds in the inhibition of aggregation of protein associated with neurodegenerative disease.